1. Field of the Invention
This invention is concerned with the manufacture of aromatic compounds having six to eight carbon atoms, namely benzene, toluene and xylenes (BTX). More particularly, the invention is directed to a high temperature isomerization process conducted in the presence of the ZSM-22 zeolite.
2. Description of the Prior Art
P-xylene is a valuable chemical feedstock which may be separated for use in synthesis of polyesters from mixed xylenes by fractional crystallization. Benzene is a highly valuable product for use as a chemical raw material. Toluene is also a valuable petrochemical used as a solvent, in chemical manufacturing processes and as a high octane gasoline component.
Typically, p-xylene is derived from mixtures of C.sub.8 aromatics separated from such raw materials as petroleum naphthas, particularly reformates, usually by selective solvent extraction. The C.sub.8 aromatics in such mixtures and their properties are:
______________________________________ Freezing Boiling Density Lbs./ Point .degree.F. Point .degree.F. U.S. Gallon ______________________________________ Ethylbenzene -139.0 277.0 7.26 P-xylene 55.9 281.0 7.21 M-xylene -54.2 282.4 7.23 O-xylene -13.3 292.0 7.37 ______________________________________
Calculated thermodynamic equilibria for the C.sub.8 aromatics at 850.degree. F. are:
______________________________________ Wt. % ethyl benzene 8.5 Wt. % para xylene 22.0 Wt. % meta xylene 48.0 Wt. % ortho xylene 21.5 TOTAL 100.0 ______________________________________
Principal sources of such raw materials are catalytically reformed naphthas and pyrolysis distillates. The C.sub.8 aromatic fractions from these sources may vary quite widely in composition, but they usually comprise about 10 to about 32 wt. % ethylbenzene, with the balance, xylenes, being divided approximately as 50 wt. % of the meta, and 25 wt. %, each of the para and ortho isomers.
Individual isomer products may be separated from the naturally occurring mixtures by appropriate physical methods. Ethylbenzene may be separated by fractional distillation, although this is a costly operation. Ortho-xylene may be separated by fractional distillation, and it is so produced commercially. Para-xylene is separated from the mixed isomers by fractional crystallization or by selective adsorption (e.g., the Parex process).
As commercial use of para and ortho xylene has increased, isomerization of the other C.sub.8 aromatics to produce an equilibrium mixture of xylenes, and thus increase the yields of the desired xylenes, became increasingly important. Octafining is one of the processes which produce an increased amount of xylenes.
In a typical plant utilizing the Octafining process, a mixture of C.sub.8 aromatics is introduced to an ethylbenzene tower wherein the stream is stripped of a portion of its ethylbenzene content, to an extent consistent with retaining all the xylenes in the feed stream without unduly expensive "superfractionation". Ethylbenzene is taken off as overhead, while a bottom stream, consisting principally of xylenes, together with a significant amount of ethylbenzene, passes to a xylene splitter column. The bottoms stream from the xylene splitter, comprising primarily ortho-xylene (o-xylene) and C.sub.9 aromatics, passes to the o-xylene tower from which o-xylene is taken off as overhead, and heavy ends are removed. The overhead from the xylene splitter column is transferred to conventional crystallization separation. The crystallizer operates in the manner described in Machell, et al., U.S. Pat. No. 3,662,013, dated May 9, 1972, the entire contents of which are incorporated herein by reference.
Because its melting point is much higher than that of the other C.sub.8 aromatics, para-xylene (p-xylene) is readily separated in the crystallizer after refrigeration of the stream, and a xylene mixture lean in p-xylene is transferred to an isomerization unit. The isomerization charge passes through a heater, is admixed with hydrogen, and the mixture is introduced to the isomerizer.
Isomerized product from the isomerizer is cooled and passed to a high pressure separator from which separated hydrogen can be recycled in the process. The liquid product of the isomerization passes to a stripper from which light ends are passed overhead. The remaining liquid product comprising primarily C.sub.8 + hydrocarbons is recycled in the system to the inlet of the xylene splitter.
It will be seen that the system is adapted to produce quantities of p-xylene from a mixed C.sub.8 aromatic feed containing all of the xylene isomers plus ethylbenzene. The key to efficient operation to accomplish that result is the use of the isomerizer which takes crystallizer effluent lean in p-xylene and converts the other xylene isomers in part to p-xylene for further recovery at the crystallizer.
Among the xylene isomerization processes available in the art, Octafining was originally unique in its ability to convert ethylbenzene. Other xylene isomerization processes have required extremely expensive fractionation to separate ethylbenzene from other C.sub.8 aromatic fractions. As set forth in the table of properties above, the boiling point of ethylbenzene is very close to those of p and m-xylene. Complete removal of ethylbenzene from the charge by conventional methods, e.g., distillation, is therefore impractical. The usual expedient for solving the problem involved the use of an ethylbenzene separation column in the isomerizer-separator loop when using catalyst other than those used in Octafining. However, Octafining does not require the use of this expensive auxiliary equipment to prevent build up of ethylbenzene in the loop. This advantageous feature is possible because the Octafining catalyst converts ethylbenzene to xylenes.
In Octafining, ethylbenzene reacts through ethyl cyclohexane to dimethyl cyclohexanes, which in turn equilibrate to xylenes. Competing reactions are disproportionation of ethylbenzene to ethane and benzene, and hydrocracking of alkyl cyclohexanes.
A significant improvement of the Octafining process arose with the introduction of zeolite catalysts, such as zeolite ZSM-5, combined with a metal, such as platinum, as described by Morrison, in U.S. Pat. No. 3,856,872. At temperatures of about 700.degree.-800.degree. F., ethylbenzene is converted by disproportionation over ZSM-5 type catalyst to benzene and diethylbenzene. At higher temperatures, and in the presence of a ZSM-5 catalyst of reduced activity, ethylbenzene and other single ring aromatics are converted by splitting off side chains of two or more carbon atoms as described in U.S. Pat. No. 4,188,282.
These developments permit upgrading of Octafining reactors by the substitution of the improved (ZSM-5) catalyst.
In the known processes for accepting ethylbenzene to the loop, conversion of that compound is constrained by the need to hold conversion of xylenes to other compounds to an acceptable level. Thus, although the Morrison technique provides significant advantages over Octafining in this respect, operating conditions are still selected to balance the advantages of ethylbenzene conversion against the disadvantages of xylene loss by disproportionation and the like.
A further advance in the art is described in patents to Morrison and Tabak, directed to various techniques for reducing acid activity of ZSM-5 zeolite catalyst, and use of such low activity catalysts for xylene isomerization concurrently with ethylbenzene conversion at temperatures upwards of 800.degree. F. For example, U.S. Pat. No. 4,163,028, the entire contents of which are incorporated herein by reference, discloses xylene isomerization and ethylbenzene conversion at high temperature with the ZSM-5 zeolite of very high silica/alumina ratio, whereby the acid activity of the catalyst is reduced.
The inventions of those patents are predicated on discovery of combinations of catalyst and operating conditions which decouple ethylbenzene conversion from xylene loss in a xylene isomerization reaction, thus permitting the use of C.sub.8 fractions, which contain ethylbenzene as the feed without sacrifice of xylenes to conditions which will promote adequate conversion of ethylbenzene. These results are obtained by the use of a catalyst characterized by zeolite ZSM-5 substantially reduced in activity, e.g., by dilution, steaming, very high silica/alumina ratio, base exchange with alkali metal, coking or the like. At the high temperatures of 800.degree.-1000.degree. F., the zeolite of reduced activity exhibits effective power for isomerization of xylene and for splitting off alkyl side chains of two or more carbon atoms from single ring aromatics at long on-stream periods. The disproportionation activity of the zeolite is severely depressed by the reduced acid activity, resulting in low losses of xylene by that mechanism. That lack of disproportionation activity impairs the capacity of the catalyst to handle trialkyl aromatics of nine or more carbon atoms, e.g., trimethylbenzene, as practiced in some processes. It thus becomes necessary to remove from the recycle stream those components having more than eight carbon atoms to avoid excessive build-up in the system of C.sub.9 and higher hydrocarbons. The catalyst also has the capacity to crack paraffins in the change to lower boiling compounds readily removable from recycle streams by fractionators normally present in the p-xylene recovery/isomerizer loop.
By reason of this combination of activities, the catalyst may be used in a system charging reformate without removal of paraffin hydrocarbons, as described in U.S. Pat. No. 4,211,836.
The catalysts of zeolite, plus a metal, such as platinum discussed above, are of the type known as "dual function catalysts" characterized by the provision of catalyst sites of different functions, each of which separately performs its function, often one step for each type of site in a multi-step reaction sequence. Such catalysts and the sequential reaction steps catalyzed by different sites are discussed and explained by P. B. Weisz, "Polyfunctional Heterogeneous Catalysis," Advances in Catalysis, 13, pp 137-190 (1962). Weisz describes some experiments in which the two types of sites are provided by separate entities, such as physical mixtures of particles each of which provides only one type of catalytic site. Isomerization of certain paraffins over physical mixtures of acidic silica-alumina and platinum on a carrier is specifically described.